1. Field of the Invention
The present invention relates to a semiconductor integrated circuit comprising a voltage conversion circuit and a bias circuit for obtaining a voltage change from a change in the capacitance of a capacitor used in, for example, an electric microphone.
2. Description of Related Art
FIG. 7 is a circuit diagram of a voltage conversion circuit according to related art for obtaining a voltage change resulting from a change in the capacitance of a capacitor where the voltage conversion circuit is part of an amplifier circuit. As shown in FIG. 7, this amplifier circuit 100 comprises a voltage conversion circuit 101, coupling capacitor 102 and amplifier 103. An electric microphone 105 (microphone below) in which the capacitance of a capacitor is changed by sound waves is connected between ground GND and the input terminal IN of the amplifier circuit 100.
The capacitor of the microphone 105 is precharged, and the capacitance of the capacitor changes with the sound waves picked up by the microphone 105 thus causing the output voltage of the microphone 105 to vary according to the capacitance change. Note that the output voltage from the microphone 105 is applied to the input terminal IN of the amplifier circuit 100.
The voltage conversion circuit 101 voltage converts the voltage Vin input to the input terminal IN, and passes the converted voltage through coupling capacitor 102 to the amplifier 103. The amplifier 103 then outputs the amplified voltage from output terminal OUT. The voltage conversion circuit 101 uses a depletion type n-channel FET 111 and resistor 112 to voltage convert the voltage Vin input to the input terminal IN. The node between the gate and source of FET 111 is biased by the bias circuit of diodes 113 and 114, and the voltage Vgs between the gate and source of FET 111 fluctuates around 0 V. The drain current Id of the FET 111 is proportional to the square of pinchoff voltage Vp.
The relationship between pinchoff voltage Vp and the drain current Idss when the gate-source voltage Vgs is 0 V can be obtained from the following equation (a): EQU Idss=.beta..times.Vp.sup.2 (a)
where .beta. is a coefficient determined by the gate size of the FET 111.
If the change in voltage Vin resulting from a change in the capacitance of the microphone 105 capacitor is .DELTA.Vin, then the change .DELTA.Id in the drain current Id of FET 111 caused by .DELTA.Vin when Vgs=0 can be obtained from the following equation (b). EQU .DELTA.Id=-2.times.Idss.times..DELTA.Vin/Vp (b)
The following equation (c) can therefore be derived from the above equations (a) and (b). EQU .DELTA.Id=-2.times..DELTA.Vin.times..beta..times.Vp (c)
Thus, if the resistance of resistor 112 is R, the change .DELTA.Vr in voltage drop Vr due to resistor 112 when there is a .DELTA.Id change in the drain current Id can be obtained from the following equation (d). EQU .DELTA.Vr=.DELTA.Id.times.R=-2.times..DELTA.Vin.times..beta..times.Vp.times .R (d)
If R=Vp/(-2.times.Idss), then we know from the above equations (b) and (d) that .DELTA.Vr=.DELTA.Vin.
With respect to the dc characteristics of the voltage conversion circuit 101, if Vx is the potential at point X, Vx will be the supply voltage Vdd minus the voltage drop of the resistor 112, and can be expressed as shown in equation (e) when current Idss flows to resistor 112. EQU Vx=Vdd-R.times.Idss=Vdd-R.times..beta..times.Vp.sup.2 (e)
When the amplifier circuit 100 is an IC device, however, variations during the manufacturing process produce variations in the pinchoff voltage Vp of FET 111. We know from equation (d) that the change .DELTA.Vr in the voltage drop Vr varies in proportion to the pinchoff voltage Vp, and, as a result, from equation (e) that the potential Vx at point X varies.
Variations during the manufacturing process also produce variations in the absolute value of the resistance R of resistor 112, and we know from equation (d) that the change .DELTA.Vr in the voltage drop Vr varies in proportion to this resistance R. Furthermore, resistance R and coefficient .beta. also have a temperature characteristic, which produces variation in potential Vx at point X.
A problem with the related art described above is therefore that a stable voltage gain and output voltage range cannot be obtained in the output voltage of the amplifier circuit 100.
In addition, the output voltage Vout from the output terminal OUT is easily saturated, and a large amplification factor cannot be achieved in the amplifier 103, due to variations in potential Vx at point X. It is therefore necessary for a coupling capacitor 102 to cut the dc component of the output voltage from the voltage conversion circuit 101, and then amplify by means of the amplifier 103. However, the output voltage from the voltage conversion circuit 101 cannot be dc amplified by the amplifier 103, and a high capacitance coupling capacitor 102 is required, making it difficult to integrate the amplifier circuit 100.